Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

========

https://hg.mozilla.org/integration/gaia-central/rev/8d15cd3ac0d0
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Merge pull request #16393 from timdream/lockscreen-backgroundsize

Bug 971654 - Correct lockscreen background sizing, r=snowmantw

========

https://hg.mozilla.org/integration/gaia-central/rev/689dd440bf7c
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Bug 971654 - Correct lockscreen background sizing
Bumping manifests a=b2g-bump